The nitrogen oxide content of exhaust from an internal combustion engine can be reduced by recirculating or recycling a portion of the exhaust into the intake system of the engine. This exhaust for recycle can be obtained from any portion of the exhaust system of the engine; and various means are available for controlling the amount of exhaust which is recirculated.
It has now been discovered that by cooling the recycle exhaust before it is introduced into the intake system, an additional reduction in nitrogen oxide content of the exhaust is unexpectedly achieved; and an additional benefit is obtained since the cooling device removes other material suspended in the exhaust. Preferably, the exhaust is cooled to a temperature below about 250.degree. F. A useful temperature range is about 150.degree. F to about 250.degree. F.